This invention relates to digital radio communication, and more particularly to a radio communication system with transmitted pilot tone for calibration and coherent demodulation of data without the use of a conventional tracking loop.
Due to the lack of substantial demand for low cost, high performance digital signal transmission in mobile links, until recently the mobile communications industry has made very little attempt to improve the poor characteristics of traditional mobile digital radios. In fact, the approach which has been widely practiced so far is brute force which wastes bandwidth and power to combat the extremely hostile propagation medium representative of mobile links. However, the worldwide upsurge in the mobile communication market of recent years has made it clear that conventional means of mobile data transmission with their poor spectral usage cannot meet the challenging needs of the spectrally limited mobile networks of the future. Conventional digital radios such as digital pagers and mobile/portable data terminals possess an extremely poor channel utility. The channel throughput of such systems in bits/s/Hz has traditionally been about 0.1 or lower. In some new designed systems the spectral efficiency of digital transmission in mobile/portable channels has been elevated to about 0.2 bits/s/Hz which.
In the late seventies a new concept in mobile communications which utilizes a satellite repeater for mobile/portable applications was developed by NASA in the U.S. and the Department of Communications in Canada. This new mode of mobile communication not only demands high channel throughput due to limited available bandwidth but also requires good link power efficiency because the power consumption of a satellite is an important factor in determining its cost. Therefore the next generation of mobile radios must demonstrate attractive power behavior as well as superior spectral utility. However, empirical as well as analytical studies indicate that due to presence of multipath fading, propagation characteristics of mobile links experience rapid variations. Such channel variations infest the transmitted signal by generating random amplitude and phase modulation whose bandwidth equals twice the doppler. Clearly, in a mobile link, the doppler shift depends on the vehicle speed and the waveform frequency. At 800 MHz, for example, doppler values of 100 Hz or higher are not uncommon.
In principle, it is desirable to reduce the time dependent channel impairments by means of adaptive calibration techniques; however, due to the large bandwidth of these impairments, traditional equalization techniques are not effective. Therefore, conventional digital radios inherently suffer from the following limitations:
1. Since fast acquisition and stable tracking of carrier phase is not easy to achieve in fading links, coherent demodulation techniques suffer from degraded carrier tracking performance when used in mobile radios. PA1 2. Noncoherent or differentially coherent techniques which are used as substitutes to coherent signal demodulation, such as differential Gaussian minimum shift keying (GMSK), generally suffer from losses exceeding 3 dB. PA1 3. Nonconstant envelope signaling techniques which are usually very bandwidth efficient are not easily used in mobile links because of the amplitude fading impairment. PA1 4. Due to the time varying random phase modulation inherent in mobile links, digital communication suffers from a link dependent error floor. This irreducible error rate is a function of doppler and the transmitted bit rate.